Current Sink or Swim Update
It’s been a while since my last update on my progress through the Contextual Electronics courses. As I’m sure everyone is familiar with life got a bit busier than expected for a couple of weeks and I had to step away from the project. Since then I have completed the schematic layout and the PCB design. I sent the boards off to OSHPark and have placed an order on Digi-Key for the components.
For now I decided to keep the overall project scope more or less the same as the original project. I ended up going through the overall process of schematic design, component selection and PCB layout probably 3-4 times as I learned a bit more each time and did a lot of independent research.
In hindsight I probably should have taken the results from the first or second iterations and gone ahead with the build and gone back for a second iteration that built on the first one. As well as posting updates more regularly to avoid getting too far down the rabbit hole trying to figure everything out on my own.
It’s worth noting that I choose to alter the design for a 2.5V supply when self powered by a single cell battery which did pose some challenges with the component selection especially with the Mosfet and the voltage regulator.
This series of FETs from STM was one of the only parts that I could find in a TO-220 package that would turn on with 2.5V at the gate and be able to handle a significant amount of current at the drain. It seemed like most of the Id/Vgs curves dropped sharply as Vgs dropped below 3.0-3.5V. Admittedly I haven’t had much experience reading Mosfet datasheets and may be missing something or mis-interpreting them.
While I was researching this looked to be a really common and readily available general purpose Op-Amp and it meet the specs so it looked to be a pretty easy choice.
I struggled finding an appropriate voltage regulator for the project. I think I created most of this problem for myself because I was hoping to incorporate an MCU for logging and the ability to monitor voltages and currents without requiring a multimeter or two while still keeping the device self powered. This probably would have been possible without going to a regulator with this much current capacity but I neglected to do much research on an appropriate MCU and likely power usage before speccing this component. Since I ended up not going this far for the first iteration it is definitely overkill. Next time I need to spec the rest of the key components for the project and do the power requirement estimations before trying to spec the power supply solution!
Using Digi-Key primarily there weren’t many potentiometers coming up in my search that bridged the gap between expensive audio centered models and simple trim pots. I knew I wanted something that I could use without a screwdriver for frequent adjustment so the trim pots were out of the question. I didn’t spend too much more time on this selection and just went with one of the cheapest options that fit the resistance range and no screwdriver requirement.
The heatsink was choosen solely on the price and ability to dissipate a minimum of 10 watts at ambient.
Current Sense Resistor: 0.1ohm 2512 SMD
I did end up going with an SMD component for the current sense resistor because it seemed to have a bit of smaller overall footprint than a comparable through hole component.
The schematic seemed to go fairly smoothly. My main edits as I went through this process was how I named a few of the Nets as I realized that some of my original naming choices made things a little bit confusing during the PCB layout process. I did use the hierarchical sheets for the opportunity to learn how they worked in KiCad.
This was where I spent a lot of my time. I spent a lot time trying to understand some of the best practices for routing traces, making multiple connections, and sizing both tracks and vias. I tore up and re-did the PCB layout a couple of times and each one of them got a little bit better than the last.
One of the things that helped everything click was maintaining a consistent orientation on the components. On my first attempt and a half I had the Op-Amp positioned with the leads on the top and bottom and the majority of the other components running horizontally on the top layer. This made routing a lot of the traces particularly challenging. It came together much easier once I rotated the Op-Amp to also be horizontal. The other key piece was making sure that my power inputs, power supply and the necessary input on the Op-Amp were somewhat in line which cleaned up and shortened the power routing significantly.
Overall I’m pretty happy with the final product. I would appreciate any feedback anyone has to offer I should probably start setting deadlines for myself on these personal projects otherwise I’m likely to never finish most of them as it’s never quite the way I want it.
Overall the project has been a lot of fun to work on so far and I’m excited to get the boards and the components in soon to start the build and test phases. I’m looking forward to continuing with the other projects in CE and starting to apply what I’ve learned to my own projects. I’m still considering another iteration of this project to add in logging of the discharge time and potentially a charge cycle to create a tool capable of characterizing battery performance.